Apparatus and method for continuously manufacturing steel sheets coated with protecting metal layers



United States Patent 72] Inventors Kazuo Matsudo;

Toshio Omori; Takayoshi ;Sh i n omura; Takashi Kamata; Toshio Kaneko; Toshio Takishima; Hiroshi Miwa, and Yasuo Koike,

Kawasaki-shi, Japan [21] Appl. No. 752,418 [22] Filed June 25, 1968 Continuation of Ser. No. 536,855, March 23, 1966, abandoned. [45] Patented Oct. 27, 1970 [73] Assignee Nippon Kokan Kabushiki Kaisha Tokyo, Japan [32] Priority March 25, 1965, July 30, 1965, March 22,

1966 [33] Japan [31] m 17,041, 40/45,836 and 41/17,343

[54] APPARATUS AND METHOD FOR CONTINUOUSLY MANUFACTURING STEEL SHEETS COATED WITH PROTECTING METAL LAYERS 1 Claim, 8 Drawing Figs.

[52] US. Cl 118/67,

m1 sesame [51] Int. Cl 1105c 11/00 [50] Field of Search 1 18/67, 66, 72, 419, 69; 26613, 5, 4(A); l48/ (Inquired) [56] References Cited UNITED STATES PATENTS 737,3.61 8/1903 Daniels 1,930,601 10/1933 Townsend 2,646,231 7/1953 Frielinghaus 3,056,694 10/1962 Mehler et a1 3,328,015 6/1967 Becker 266/5 Primary Examiner-John P. McIntosh Attorney-Sughrue, Rothwell, Mion, Zinn and Macpeak ABSTRACT: A strip steel coating method and apparatus where strip steel is coated in a molten bath and subsequently fed through conditioning apparatus to windup reels having a heat treatment chamber provided around the reels to relieve stresses induced in the strip prior to reeling. Conveying means are provided in the heat treatment chamber to move the rolls of strip steel through the treatment chamber which consists of a heating chamber and a cooling chamber.

Patent ed 0a. 27, 1970 Sheet f lb zmc OVERAGING PERIOD (LOGARITHMIC) TEMP, 300C Sheet m l- E Patented Oct. 27, 1970 3,536,036

' Sheet 4 of 4 J"T-.5B

I 500- uJ 3 400 55 .i r 5 lb 50 I6 T TIMEMINUTES SCALE: LOQHRITHMW 500- 400 IT n E E5 300 LU Q. E 200- 1.-

loo-

i 5 lb 50 .6

TIME, M\NUTES SCALE LOQARTHMIC APPARATUS AND METHOD FOR CONTINUOUSLY MANUFACTURING STEEL SHEETS COATED WITH PROTECTING METAL LAYERS This application is a continuation of Ser. No. 536,855 filed Mar. 23, 1966, now abandoned.

This invention is an apparatus for carrying out continously the dip-coating of elongated metal strips with molten metal. More specifically, the invention relates to an improved apparatus for continously coating elongated cold-rolled metal strips or hands of low carbon steel, with molten metal or salts thereof, having lower melting point, such as aluminium, zinc or the like.

Considerable progress has been recently made in the field of the above kind of continous molten metal coating technique on low carbon steel strips. For carrying out this process, the annealing for softening purpose is of grave importance.

In some conventional plants utilized for this purpose, the annealing step for the strip is provided within the line. Other conventional plants are also provided with such annealing means as before, which are however arranged outside the processing line. As ascertained, however, from our practical experience that when relying upon any of these conventional plants, the softening effect obtainable through the way of annealing is not so much effective that the final sheet products can not be utilized for deep-pressing jobs as frequently employed in the automotive industries and the like.

The reason may be attributable to the fact that the steel strip, upon dippingly coated, is forcibly passed through a forced cooling stage for suppressing the generation of secondary alloy layer on the strip to a medium degree, and for protecting the coated layer from possible damages or scaling-off caused by an abrupt change of the traveling direction of the strip, and by otherwise considerable temperature difference as cold deflector rolls serving for the deflecting service. For guidance in this respect, the temperature of the strip directly after being subjected to the dip coat- 460C and 670C in the case of zinccoating and aluminium-coating process, respectively.

According to a recent proposal as revealed by W.C. Leslie et al., in the 6th A.I.M.E.-Convention, held at Chicago, 31st Jan, 1964, and known as the post-galvanizing anneal process, the annealing is carried out at 370C for 4 hours. As ascertained by our experimental results executed in line with the above proposal, such a long continued annealing led transforming the coated layer substantially as a whole into the secondary alloy coating, so far as zinc-coated steel strips are concerned.

It is therefore the main object of the invention to provide a continuous plant for the manufacture of dip-coated steel strips, especially zincor aluminium-coated one, capable of being utilized in deep-pressing jobs by reason of well softened stock quality through ideally improved continuous annealing.

Another object is to provide a manufacturing plant of the above nature, capable of performing the aforementioned softening anneal at a certain selected place along the manufacturing line.

A still another object is to provide a plant of the above kind which is of lower investment costs, reliable in its operation and operable at an economical manner.

These and further objects, features and advantages of the present invention will become clear when read the following detailed description in reference to the accompanying drawings illustrative of a preferred embodiment of the invention, as well as several reference diagrams and miscroscopic photographs.

In the accompanying drawings:

FIG. 1 is a schematic side view of an embodiment of the plant according to the invention, yet intentionally omitting an intermediate chamber and a final cooling chamber for clarity of the drawing.

FIG. 2 is an enlarged vertical sectional view of an overaging chamber with its attached intermediate chamber.

FIG. 3 is a schematic horizontal section of an assembly arranged along the end part of the processing line of the plant and comprising the overaging chamber, the intermediate chamber and the finaling cooling chamber.

FIG. 4 is a cross-sectional view taken along a line Ill-IV in FIG. 2,

FIGS. 5A and 5B are diagrams showing relationship between the overage treatment and the formation of secondary alloy layer in the protecting coating on a steel strip as processed through the plant according to the invention, in the case if zincand aluminium-coating modes.

FIG. 6 is a diagram showing a variation behavior of quench aging exponent as measured in the course of an overaging treatment carried out in the overaging chamber, in the case of zinc-coating.

FIG. 7 is a similar diagram to FIG. 6, plotted from the results in the case of aluminium-coating process with use of the plant according to this invention.

Now referring to the accompanying drawings, especially in FIGS. 1-4, numeral 1 denotes a conventional and stationary heat treating furnace adapted for reducingly heating a steel strip 9 and comprising a substantially closed housing la, in which a turn-down roll 2 is freely rotatably mounted. A guide lower level than the furnace 1. In the molten bath 5 in the pot 4, there is provided a sink roll 6 which is rotatably mounted within the pot and adapted for being driven from a suitable prime mover such as an electric motor, although the bearing means and the driving means for the roll 6 have been omitted from the drawing for simplicity thereof and on account of very familiarity.

Above the roll 6, there is provided a wiper or a pair of pinch rolls 7 which are also rotatably mounted on the housing 4a, although the bearing means are omitted from the drawing again for simplicity.

At a proper height from the level of pot 4, there is provided a steam jet device 8 which is mounted on a stationary member, not shown and arranged at a suitable place along the scheduled passage of the band stock at 9' to be coatingly treated, said device 8 serving as conventionally for use in still higher level, a cooling hood l0 and a deflector roll 11 are provided, the latter being freely rotatably mounted in a stationary bearing means which has been again omitted from the drawing for simplicity. The cooling hood 10 is fitted with compressed air supply means with nozzles, for effectively cooling the travelling stock at 9, although at a further forward place along the travel passage of a strip stock 9", a group of positively driven bridle rolls [2, arranged to be adapted for generating and maintaining a proper tension in the stip as a whole. At a properly selected distance from the bridle roll group 12, there is a series of looper rolls 13 which are mounted in a freely rotatable manner. Several dotted lines are shown in FIG. 1, illustrative of an alternative and additional cal bath 15, the latter containing preferably and by way of example a body of chromate solution for chemically conditioning the surface conditions of the coated layer on the steel strip, as will be described hereinafter. Leveller l4, and looper l3 serve, as is commonly known, as straightening means. Both devices 13 and 14 comprise each a plurality of positively driven rolls for effectively serving for the desired straightening purpose. Looper 13 comprises generally two groups of rolls, one of which groups is mounted on a shiftable looper mount, only schematically shown, in FIG. 1 and in practice designed preferably in the form of carriage 16 which is mechanically connected with a driving means 17, adapted for shifting and positioning the carriage at an adjustable and properly selected position so as to meet occasional demands for maintaining the strip tension at a properly selected value. There are provided a suitably selected number of guide rolls 18-21 for guiding the strip along its scheduled travel passage as shown.

In the case of zinc-coating mode of the operation, the steel strip, upon preliminarily heat-treated to, say 500C, for instance, is led from roller 2 in the furnace I kept preferably at 650C by way of example, to the dipping pot 4, the bath of which is kept preferably between 450-460 C, wherein the strip is coated with a thin layer of the molten metal 5, wiped off surplus molten metal while passing through between pinch rollers at 7. whereupon, the coated strip at 9, generally having a temperature of 470C, for instance, (670C in the case of aluminium-coating process) is subjected as conventionally to a minimum spangle treatment at 8, by injecting steam jets upon the strip surface. The strip is then passed through the forced-cooling hood 10 so as to be abruptly quenched to, say below 100C. By this quench treatment, the hardness of the strip will be considerably increased by reason of the quenching effect, thus the carbon in steel is turned into the form of solid solute to an oversaturated state.

In the conventional art, the thus coated and quench hardened steel strip is fed through deflector roll 11, bridle rolles 12, leveller l4, chemical conditioner and guide rolls 18-21 to looper 13, thence through passage at 9" to conventional tension reels, 27-28, to be wound up therearound in the form of strip coils. These strip coils may not be well utilized in deep-pressing jobs, on account of deficiency of desired softness of the steel.

In the present galvanizing plant, the coated strip is directly fed from bridle rolls to the looper 13. The conventional leveller and the chemical conditioner may be removed from the processing line, or alternatively kept opened without use.

The coated strip, while being subjected to control action provided by the control unit 16-17 so as to meet with occasional variations in the strip guiding conditions, and kept at a predetermined temperature between, say, 60C and the normal temperature, is fed through deflectors 23 and bridle rollers 24 to preheating chamber 22, and thence to overaging chamber 25, which is the most important constituent of the present improved plant, said overaging chamber being preferably kept at 300-330C in the case of zinc-coating purpose, or 300-350C in the case of aluminium-coating purpose. The both chambers 22 and 25 may be united into one and arranged so that part of the heating fluid in the overaging chamber is recirculated in the preheating chamber. As shown, the exhaust from the preheating chamber 22 is led to a heater 26 wherein it is heated to the aforementioned, specified temperature, for compensating the working heat loss. As the heating medium, any selected inert gas, such as gaseous nitgrogen may preferably employed. Under circumstances, however, air may be utilized for the same purpose, when a slightest oxidation of the protecting layer on the steel strip may be tolerated. For avoiding an excessive escapement of the heating medium from the preheating chamber, there is provided sealing rolls 26 at the entrance of the chamber 22, through which the strip to be overaged is guided and introduced. This provision was made, for rather preventing the escapement of the heating medium as hinted above, from taking place, and at the same time for preventing secondary air from invading into the chamber unit, for keeping the thermal efficiency at a possible maximum. The problem of prevention of possible oxidation is of rather least significance, because at the above-specified overaging temperature, practically any oxidation of the coated layer or steel per se need not be feared.

There are provided pivotable mechanical shifters 29-30 within the interior space of the overaging chamber so as to cooperate with respective tension rollers 27-28, for guiding the'fed strip thereto. When roller 27 is pivotally horizontal position, not shown, the strip is led to the second roller 28, and vice versa.

When a predetermined length of the processed strip is wound up by either tension reel 27 or 28, a shearing device at 31, which may be either up-cut or down-cut type, will be automatically operated as conventionally, so as to cut the strip off the wound strip coil 34 or 35. Although the details are not shown on account of its highly familiamess, mandrels or tension reels 27-28 are of collapsable type. When the strip is cut off by means of the shearing device 31, either magnet lifter 32 or 33 is energized so as to attract the strip coil after the reel 27 or 28 has been collapsed to reduce its diameter. The thus attracted strip coil 34 (or 35) is then conveyed from left to right when seen in FIG. 4 to a position as at 34'. Next, the magnet lifter is deenergized so that the shifted coil 34' will drop onto coil conveyer 36 arranged within conveyer pit 37, thence conveyed to intermediate chamber 38. The strip coil is kept maintained at the overaging coil temperature, 300C for instance, for about 30 minutes in total. The drive mechanism for the conveyer has been omitted from being shown and described, on account of its high similarity. Operable doors 39 and 40 are provided so as to separate the intermediate chamber from overaging chamber 22 and cooling chamber 41, respectively. The conveyer belt system 36 with a number of conventional sadles for mounting the processed strip coils as schematically shown, extends through the intermediate chamber into the cooling chamber 41 as well as discharge chamber 42 connected therewith through the agency of a closable door 43. The cooling medium prevailing within the cooling chamber is recirculated through a circulating system 44 fitted with a heat exchanger 45 which is in turn connected with a refrigerator 46 for extracting the waste heat extracted from the strip coils as the final products through the intermediary of the cooling medium which way again preferably gaseous nitrogen, carbon dioxide, air or the like. The cooled products are then successively discharged therefrom by passage through the discharge chamber 42 while being conveyed by the conveyer system 36. Numerals 47 and 48 designate belt-wrappers adapted to wrap the coming end of the processed strip onto the respective tension rolls 27 and 28, respectively. which construction and function are highly familiar to those skilled in the art so that no further detailed description thereof would be necessary to reveal.

In FIG. 5A, curve a shows an overaging characteristic curve relative to zinc. This kind of curve is specific to occasionally adopted temperature. In this instance, this curve has been plotted from the results of a large number of experiments carried by us, this curve a being prepared based on a preferred temperature of 300C. In the area below this curve, the solid solute carbon in steel is in the way of depositing. Along this curve, the solid solute carbon has been completely separated out, which means a threshold of softening treatment, on the other hand, in the area above this curve, no softening effect can be obtained.

Curve b in FIG. 5 show a characteristic curve demonstrating the tendency in the development of secondary zinc alloy in the coated layer. If a softening effect be obtained without affecting upon the zinc coating layer, the annealing temperature should be selected from the area below this curve b In FIG. 5B, curves a and b has been plotted in connection with aluminium in place of zinc and correspond to those shown in FIG. 5A by a and b respectively.

FIG. 6 is a measure of quench hardening effect on zinccoated steel sheet, for better guidance to select the annealing conditions in the said chamber 22.

FIG. 7 shows an overaging characteristic curve plotted in connection with aluminium as the coating material and is similar to FIG. 6.

We claim:

1. Apparatus for continuously manufacturing steel strips coated with protecting metal layers such as zinc, aluminum and the like, comprising a reducing furnace for subjecting a steel sheet stock to a reducing treatment on the surface thereof, a pot containing a bath of fused metal to coat said strip fed from said furnace, and a post conditioning arrangement for improving the softness of steel in the thus coated strip as well as the nature of the coating layers including at leat one reel for coiling said coated strip characterized by the cooling chamber connected with said intermediate chamber for receiving and cooling said coated and heated strip, means for supplying a cooling gas to said cooling chamber, means for conveying said coiled coated strip from said heating chamber through said intermediate chamber to said cooling chamber and said heat treatment chamber being provided with additional conveying means for moving said coiled coated strip from said reel to said first mentioned conveyor. 

